Operation of semiconductor devices typically depends upon movement of charge carriers through portions of the device structure. Accordingly, the lower the mobility of charge carriers in a structural feature, the slower the device may function. One technique intended to increase charge carrier mobility includes increasing crystal lattice strain in semiconductive material. For example, conventional efforts include a variety of methods that increase tensile stress and thus increase charge carrier mobility. Other efforts to increase charge carrier mobility include substituting silicon in MOS devices with germanium. Germanium exhibits a lower band gap (0.85 eV) compared to silicon (1.1 eV), providing a higher drive current in MOS devices due to increased charge carrier mobility. However, germanium also exhibits significantly higher leakage currents and produces drain induced barrier lowering (DIBL), as known to those of ordinary skill.
Other efforts to improve transistor devices include replacement of silicon dioxide gate dielectric materials with high K gate dielectric materials. In many devices, silicon dioxide dielectric has reached its scaling limit due to leakage currents. Excessive scaling allows leakage of charge carriers through silicon dioxide gate dielectric to the gate, significantly increasing power drain and adversely affecting operation of the transistor devices. High K gate dielectric materials might be used to decrease current leakage through the gate dielectric. However, high K dielectric materials used in combination with germanium exhibit poor interface properties at the interface between the high K gate dielectric and germanium-containing channel material. The gains in charge carrier mobility when using germanium with a high K dielectric material (such as shown in FIG. 4) do not produce the expected corresponding improvement in drive current. To date, efforts attempting to integrate high K dielectric in germanium MOS devices have achieved limited success.
Accordingly, a desire exists to use germanium in transistor devices in a manner taking greater advantage of the enhanced charge carrier mobility of germanium than previously obtained. Also, a further desire exists to incorporate the advantageous characteristics of high K gate dielectric materials with the enhanced charge carrier mobility of germanium.